In recent years, in the field of optical transmission, there has been a demand of conversion technique to convert electrical signals to optical signals at a high speed due to expansion of the internet and increase of information amount to be handled. A conversion technique using a differential Electrical/Optical (E/O) converter is generally known as a high-speed conversion technique.
In the conversion technique using a differential E/O converter, an E/O converter of a transmitting optical communication apparatus converts an electrical signal to an optical signal having a waveform corresponding to a potential difference between the positive phase component and the negative phase component of the electrical signal obtained by converting an input packet by using the potential difference. An input packet may be an IP packet or the like that is input through, for example, 10G Ethernet (registered trademark). Then, the optical signal converted by the transmitting optical communication apparatus is transmitted to a receiving optical communication apparatus, and an O/E convertor of the receiving optical communication apparatus converts the optical signal to an electrical signal. The receiving optical communication apparatus determines data based on the waveform of the electrical signal obtained by converting the optical signal. For example, the receiving optical communication apparatus determines that data is “1” when the waveform of the electrical signal obtained by converting the optical signal is larger than a predetermined area and determines that data is “0” when the waveform of the electrical signal is smaller than the predetermined area (Japanese Laid-open Patent Publication No. 2012-124731).
However, in the conventional technique, improvement of a signal waveform that is deteriorated due to presence/non-presence of input packets is not considered.
That is, in the conventional technique, impedance of a capacitor of AC coupling that allows a positive phase component and a negative phase component to be input to the E/O converter becomes near zero and the midpoints of the potential of the positive and negative components match in a period where input packets to be input to the transmitting optical communication apparatus are present. On the other hand, in a period where no input packet to be input to the transmitting optical communication apparatus is present, impedance of a capacitor of the AC coupling that allows the positive phase component and the negative phase component to be input to the E/O converter becomes infinitely large and the midpoints of the potential of positive and negative components to be input to the E/O converter are apart in opposite directions. A case where the input packets are IP packets input through 10G Ethernet is assumed as an example. In this case, in an Inter-Frame Gap (IFG) that is a time period between an IP packet and another IP packet where no IP packet is present, the midpoints of the potential of an positive phase component and the midpoint of the potential of a negative phase component to be input to the E/O converter are apart in opposite directions. The E/O converter converts an electrical signal to an optical signal having a waveform corresponding to a potential difference between the positive phase component and the negative phase component that are apart in opposite directions using the potential difference. Therefore, a waveform of an optical signal output from the E/O converter of the transmitting optical communication apparatus may be deteriorated. A deteriorated waveform of an optical signal output from the E/O converter of the transmitting optical communication apparatus makes it difficult for the receiving optical communication apparatus to determine whether a waveform of an electrical signal obtained by converting an optical signal is larger than a predetermined area. Thus, the receiving optical communication apparatus may wrongly determine data.